1. Field of the Invention
The present invention relates to liquid crystal display devices, and more particularly, to display devices having a memory effect, such as ferroelectric liquid crystal panels.
2. Description of the Related Art
In previous ferroelectric liquid crystal panels described in, for example, U.S. Pat. Nos. 4,655,561, 4,836,656 and 4,844,590, a desired screen is written by selectively applying to each pixel on a selected scanning line at two different phases a voltage having one polarity and a voltage having the other polarity which are high enough to switch a pixel.
Thus, writing is conducted on the ferroelectric liquid crystal panel in accordance with the polarity of a DC pulse which is applied to the liquid crystal. It is therefore necessary for a voltage having one polarity and a voltage having the other polarity to be applied by both a scanning driving circuit for driving scanning lines and a data line driving circuit for driving data lines using a predetermined voltage as a reference. In an example of the driving method shown in FIG. 6, V.sub.1 (36 volts), V.sub.2 (0 volts) and V.sub.c (18 volts) are supplied to the scanning line driving circuit while V.sub.3 (24 volts), V.sub.4 (12 volts) and V.sub.c (18 volts) are supplied to the data line driving circuit.
The voltages supplied to the driving circuits, such as voltages V.sub.1 to V.sub.4 and V.sub.c, are generally generated on the basis of power supplied from an external power source of 100 volts (as used in Japan), 110 volts (as used in the United States), or a battery power source. The present inventors conducted experiments and found that DC voltages are applied irregularly to the liquid crystal due to a difference in the time constant between the scanning line driving circuit and the data line driving circuit. This difference in the time constant results in an image disturbance of a few (i.e., one to two) seconds immediately after the voltage supply to the scanning line driving circuit and the data line driving circuit is interrupted (i.e., power is turned off) during a writing period during which refresh (i.e., repetitive) scanning is performed on the display panel. In particular, the present inventors discovered that a DC voltage is supplied to the liquid crystal on a writing scanning line immediately before the power is turned off which is sufficiently large to disturb the uniform orientation of the liquid crystal along that scanning line.
Furthermore, it is commonly understood that a scanning signal having one polarity pulse for erasing the written state of a pixel and other polarity pulse is used advantageously in ferroelectric liquid crystal panel driving methods because it provides a sufficient driving margin, assures a fast screen rewriting speed and can be implemented by a simple control system. However, such a driving margin changes with time, as described below.